On a '74 2.0 with original D Jet-

Since my last post, got the Bowlsby rebuilt MPS in and although a bit hard to start cold the car is running better. Auxiliary Air is still blocked off, as is cold start. New TPS. Although the Aux Air seems to close, I want to do some more testing on it.

Now it seems I cannot get the idle to fall below about 11-1200 rpm. In fact, it will run with the throttle body airscrew all the way in.

Am I correct in assuming that it has to be a vacuum leak somewhere allowing air in?

I've been through virtually everything checking for leaks except the throttle body itself and the plenum. Probably the issue?

Thanks-

yes. air is getting in somehow


the vacuum leak could be in the TB itself...
I had the guy in NM rebuild my djet TB this spring
car has never idled this well
and car used to be hard to get below 1200rpm at idle.

I was wondering if anyone did that. Who rebuilds throttle bodies?

when I was researching....
everyone had positive experiences with the guy in NM

apparently he is on the samba alot


He was fast
I was super pleased with his work.
not cheap but I don't think there is anyone else... plus he seems to do it correctly the first time:

throttle body guy, in vendors section

Super pleased at $340 is a bargain. Quite a bit of work goes in to the service.

I agree...
sure its a lot of money
but also glad that such an obscure service even exists..
and its worth paying for quality
brant

I'm with you-

where would we be without specialty vendors like this?

Might (?) be the TB gaskets and/or the duct tube. Replaced gaskets; duct had a small crack, fixed with black gasket stuff and small black gorilla tape strip. Not concours, but it runs flawlessly now.

What is the TB typical failure? Is it leaking through the plate shaft or is the throttle plate itself leaking?

the shaft bushings wear out.
this ovals the shaft wear point slightly
the air leaks through the bushings
additionally this shifts the throttle plate slightly
which starts dragging in the bore and creating leak in the butterfly plate also due to the movement in the bushings.

the rebuild includes a new throttle stop that puts the closed pressure onto the stop bolt
as the original design put the stop pressure on the butterfly plate against the bore of the TB.

What do you mean duct tube? The vacuum tubes for distributor?

The ribbed duct tube just upstream of the TB (see photo). Tricky - the crack was on the underside, along a rib valley.
Click to view attachment

I’m working on a 2.0. Different set up

On a 2.0L there are also the hollow support tubes through the intake air plenum that are known to leak. Solder or seal those up.

Another test is to check the whole intake system with a smoke pencil when the engine is running, or put a shop vac in blower mode to the tailpipe (engine not running) and test joints with soapy solution to check for intake leaks.

Thanks Jeff! Did not know about the support tubes in the plenum.

I had the same problem with my '73 914 2.0L with stock D-Jet, couldn't get it to idle below 1200 rpm with the TB air by pass screw all the way in. My TB butter fly valve has a small hold in it that allows air into the engine when the butter fly valve is closed (idle). I sealed the hole (temporarily with tape) which reduced the idle rpm to a very low level (less than 500 rpm) I was then able to screw the TB air by-pass screw out until idle speed came up to 900 rpm. The idle speed is now rock steady at 900 rpm.

i was going to suggest checking this OR AF meter recheck your mix it could be still a bit lean. every MPS i’ve either rebuilt myself of has done by something else even one by Jeff still had me with higher idle , and still had to be a bit richer, that’s according to the AF and my idle and timing checks. i don’t think my MPS thst was built and calibrated by jeff needed even a 1/4 turn on the inner screw and but that’s made all the difference.